Chapter 13 Wade 8th.ppt

© 2013 Pearson Education, Inc.

Chapter 13Lecture

Organic Chemistry, 8th Edition

L. G. Wade, Jr.

Nuclear Magnetic Resonance Spectroscopy

© 2013 Pearson Education, Inc.

Rizalia KlausmeyerBaylor UniversityWaco, TX

© 2013 Pearson Education, Inc. Chapter 13 2

Introduction• NMR is the most powerful tool available for

organic structure determination.• It is used to study a wide variety of nuclei:

1H 13C 15N 19F 31P


Nuclear Spin• A nucleus with an odd atomic number or

an odd mass number has a nuclear spin.• The spinning charged nucleus generates

a magnetic field called magnetic moment.


External Magnetic Field

• An external magnetic field (B0) applies a force to a small bar magnet, twisting the bar magnet to align it with the external field.

• The arrangement of the bar magnet aligned with the field is lower in energy than the arrangement aligned against the field.


Alpha-Spin State and Beta-Spin State

• The lower energy state with the proton aligned with the field is called the alpha-spin state.

• The higher energy state with the proton aligned against the external magnetic field is called the beta-spin state.


Proton Magnetic Moments


E and Magnet Strength• Energy difference is directly proportional to

the magnetic field strength.E = h = h B0

2

• Gyromagnetic ratio, , is a constant for each nucleus (26,753 sec–1gauss–1 for H).

• In a 14,092-gauss field, a 60-MHz photon is required to flip a proton.

• Low energy, radio frequency.


Two Energy States

• A nucleus is in resonance when it is irradiated with radio-frequency photons having energy equal to the energy difference between the spin states.

• Under these conditions, a proton in the alpha-spin state can absorb a photon and flip to the beta-spin state.


Induced Magnetic Field


Magnetic Shielding• If all protons absorbed the same amount

of energy in a given magnetic field, not much information could be obtained.

• But protons are surrounded by electrons that shield them from the external field.

• Circulating electrons create an induced magnetic field that opposes the external magnetic field.


Shielded Protons

• A naked proton will absorb at 70,459 gauss.• A shielded proton will not absorb at 70,459 gauss so

the magnetic field must be increased slightly to achieve resonance.


Protons in a Molecule

• Protons in different chemical environments are shielded by different amounts.

• The hydroxyl proton is not shielded as much as the methyl protons, so the hydroxyl proton absorbs at a lower field than the methyl protons.

• We say that the proton is deshielded somewhat by the presence of the electronegative oxygen atom.


NMR Signals• The number of signals shows how many

different kinds of protons are present.• The location of the signals shows how

shielded or deshielded is the proton.• The intensity of the signal shows the

number of protons of that type.• Signal splitting shows the number of

protons on adjacent atoms.


The NMR Spectrometer


NMR Instrument


The NMR Graph

• The more shielded methyl protons appear toward the right of the spectrum (higher field); the less shielded hydroxyl proton appears toward the left (lower field).

upfielddownfield


Tetramethylsilane

• TMS is added to the sample as an internal standard.

• Since silicon is less electronegative than carbon, TMS protons are highly shielded.

• The TMS signal is defined as 0.00 ppm.

• Organic protons absorb downfield (to the left) of the TMS signal.


Chemical Shift• The variations in the positions of NMR

absorptions, arising from electronic shielding and deshielding, are called chemical shifts.

• The chemical shift (in ppm) is independent of the spectrometer used.

• Most common scale is the (delta) scale.


Delta Scale


A 300-MHz spectrometer records a proton that absorbs at a frequency 2130 Hz downfield from TMS.(a) Determine its chemical shift, and express this shift as a magnetic field difference.(b) Predict this proton’s chemical shift at 60 MHz. In a 60-MHz spectrometer, how far

downfield (in gauss and in hertz) from TMS would this proton absorb?

We substitute into the equation

Solved Problem 1

Solution


Electronegative Atoms

• More electronegative atoms deshield more and give larger shift values.

• Additional electronegative atoms cause an increase in chemical shift.


Location of Electronegative Atoms

• The deshielding effect of an electronegative substituent drops off rapidly with distance.


Typical Values


Magnetic Fields in Aromatic Rings

• The induced magnetic field of the circulating aromatic electrons opposes the applied magnetic field along the axis of the ring.

• Protons in the region where the induced field reinforces the applied field are deshielded and will appear at lower fields in the spectrum between 7–8.


Magnetic Field of Alkenes

• The pi electrons of the alkene generate a magnetic field that opposes the applied magnetic field in the middle of the molecule but reinforces the applied field on the outside where the vinylic protons are located.

• This reinforcement will deshield the vinylic protons, making them shift downfield in the spectrum to the range of 5–6.


Magnetic Fields of Alkynes• When the terminal triple

bond is aligned with the magnetic field, the cylinder of electrons circulates to create an induced magnetic field.

• The acetylenic proton lies along the axis of this field, which opposed the external field.

• The acetylenic protons are shielded and will be found at 2.5.


Deshielding of the Aldehyde Proton

• Like a vinyl proton, the aldehyde proton is deshielded by the circulation of electrons in the pi bond.

• It is also deshielded by the electron-withdrawing effect of the carbonyl (C═O) group, giving a resonance between 9 and 10.


O—H and N—H Signals

• The chemical shift of the acidic protons depends on concentration.

• Hydrogen bonding in concentrated solutions deshields the protons, so signal is around 3.5 for N—H and 4.5 for O—H.

• Proton exchanges between the molecules broaden the peak.


Carboxylic Acid Proton

• Because of the high polarity of the carboxylic acid O—H bond, the signal for the acidic proton will be at shifts greater than 10.


Number of Signals

• Methyl tert-butyl ether has two types of protons, giving two NMR signals.

• Chemically equivalent hydrogens have the same chemical shift. All the methyl groups of the tert-butyl group are equivalent, and they produce only one signal.


tert-Butyl Acetoacetate

• The spectrum of tert-butyl acetoacetate has only three signals. The most shielded protons are the methyl groups of the tert-butyl. The most deshielded signal is the methylene (CH2) because it is in between two carbonyl groups.


Using Table 13-3, predict the chemical shifts of the protons in the following compounds.

Solved Problem 2

Solution


Intensity of Signals: Integration

• The amount the integral trace rises is proportional to the area of that peak.

• The integration will have a trace for the tert-butyl hydrogens that is three times as large as the trace for the methyl hydrogens. The relative area for methyl and tert-butyl hydrogens is 1:3.


Spin-Spin Splitting• Nonequivalent protons on adjacent carbons have

magnetic fields that may align with or oppose the external field.

• This magnetic coupling causes the proton to absorb slightly downfield when the external field is reinforced and slightly upfield when the external field is opposed.

• All possibilities exist, so signal is split. This splitting of signals into multiplets is called spin-spin splitting.


1,1,2-Tribromoethane

Spin-spin splitting is a reciprocal property. If one proton splits another, the second proton must split the first.


Doublet: One Adjacent Proton

• Hb can feel the alignment of the adjacent proton Ha.

• When Ha is aligned with the magnetic field, Hb will be deshielded.

• When Ha is aligned with the magnetic field, Hb will be shielded.

• The signal is split in two and is called a doublet.


Triplet: Two Adjacent Protons• When both Hbs are aligned

with the magnetic field, Ha will be deshielded.

• When both Hbs are aligned with the magnetic field, Ha will be deshielded.

• It is more likely that one Hb will be aligned with the field and the other Hb against the field. The signal will be at its normal position.

• The signal is split in three and is called a triplet.


The N + 1 Rule

If a signal is split by N equivalent protons, it is split into N + 1 peaks.


When you see splitting, look fornonequivalent protons on

neighboring carbon atoms.


• Equivalent protons do not split each other.• Protons bonded to the same carbon will

split each other if they are nonequivalent.• Protons on adjacent carbons normally will

split each other.• Protons separated by four or more bonds

will not split each other.

Spin-Spin Splitting Distance


Long-Range Coupling

• When the hydrogen atoms are four bonds or more apart, spin-spin splitting is not normally observed. When it actually does occur, it is called “long-range coupling.”


Splitting for Ethyl Groups

• The hydrogens on the CH3 are affected by the two neighboring hydrogens on the adjacent CH2 group. According to the N+1 rule, the CH3 signal will be split into (2+1) = 3. This is a triplet.

• The hydrogens on the CH2 group are affected by the three hydrogens on the adjacent CH3 groups. The N+1 rule for the CH2 signal will be split into (3+1) = 4. This is called a quartet.


Splitting for Isopropyl Groups

• The hydrogen on the CH has six adjacent hydrogens. According to the N + 1 rule, the CH signal will be split into (6 + 1) = 7. This is called a septet.

• The two CH3bs are equivalent, so they give the same signal.

They have one adjacent hydrogen (CH), so their signal will be N + 1, 1 + 1 = 2 (doublet).


Ethyl and isopropyl groups arecommon. Learn to recognize

them from their splitting patterns.


Propose a structure for the compound of molecular formula C4H10O whose proton NMR spectrum follows.

Solved Problem 3

Solution


Solved Problem 3 (Continued)Solution (Continued)


Coupling Constants

• The coupling constant is the distance between the peaks of a multiplet (in Hz).

• Coupling constants are independent of strength of the external field.

• Multiplets with the same coupling constants may come from adjacent groups of protons that split each other.


Values for Coupling Constants


Proton NMR for p-Nitrotoluene

• para-Nitrotoluene has two pairs of equivalent aromatic protons a and b. Since the coupling constant for ortho hydrogens is approximately 8 Hz, the peaks of the signal will be separated by around 8 Hz.


Vinylic Coupling Constants

• There are two vinylic protons in 4,4-dimethylcyclohex-2-ene-1-one, and they are cis.

• The coupling constant for cis coupling is approximately 10 Hz, so the peaks should be separated by that amount.


Complex Splitting

• Signals may be split by adjacent protons, different from each other, with different coupling constants.

• Example: Ha of styrene is split by an adjacent Hb trans to it (J = 17 Hz) and an adjacent Hc cis to it (J = 11 Hz).


Splitting Tree for Styrene


1H-NMR Spectrum of Styrene


Watch for unusually large coupling constants, especially in the vinyl region,

where they may indicate the stereochemistry about a double bond.


Stereochemical Nonequivalence• If the replacement of each of the protons of a —CH2

group with an imaginary “Z” gives stereoisomers, then the protons are nonequivalent and will split each other.


Diastereotopic Vinylic Protons

• Replacing the cis hydrogen gives the cis diastereomer, and replacing the trans hydrogen makes the trans diastereomer.

• Because the two imaginary products are diastereomers, these protons are called diastereotopic protons.

• Diastereotopic hydrogens are capable of splitting each other.


Diastereotopic Protons

• The two protons on the —CH2Cl group are diastereotopic; their imaginary replacements give diastereomers.

• Diastereotopic protons are usually vicinal to stereocenters (chiral carbons).


Proton NMR Spectrum of 1,2-Dichloropropane

• Proton NMR spectrum of 1,2-dichloropropane shows distinct absorptions for the methylene protons on C1.

• These hydrogen atoms are diastereotopic and are chemically nonequivalent.


Time Dependence• Molecules are tumbling relative to the

magnetic field, so NMR is an averaged spectrum of all the orientations.

• Axial and equatorial protons on cyclohexane interconvert so rapidly that they give a single signal.

• Proton transfers for OH and NH may occur so quickly that the proton is not split by adjacent protons in the molecule.


Hydroxyl Proton in a Pure Sample

• Ultrapure samples of ethanol show splitting.


Hydroxyl Proton in an Impure Sample

• Ethanol with a small amount of acidic or basic impurities will not show splitting.


N—H Proton

• The acidic proton on the nitrogen has a moderate rate of exchange.

• Peak may be broad.


Identifying the O—H or N—H Peak• Chemical shift will depend on concentration

and solvent.• To verify that a particular peak is due to O—H

or N—H, shake the sample with D2O to exchange the H for a D.

• The deuterium is invisible in the proton NMR, so the original signal for the OH will disappear.

R—O—H + D—O—D R—O—D + D—O—H

R—NH2 + 2 D—O—D R—ND2 + 2D—O—H


Remember to look for structuralinformation based on

1. number of absorptions,2. chemical shifts,3. areas of peaks,4. spin-spin splitting.


Carbon-13 NMR

• Carbon-12 (12C) has no magnetic spin.• Carbon-13 (13C) has a magnetic spin, but is

only 1% of the carbon in a sample.• The gyromagnetic ratio of 13C is one-fourth

of that of 1H.• For carbon, a technique called Fourier

transform spectroscopy is used.


Fourier Transform NMR• Radio-frequency pulse given.• Nuclei absorb energy and precess (spin)

like little tops.• A complex signal is produced, then decays as

the nuclei lose energy. • Free induction decay (FID) is converted to

spectrum.


Carbon Chemical Shifts

• Table of approximate chemical shifts values for 13C-NMR. Most of these values for a carbon atom are about 15–20 times the chemical shift of a proton if it were bonded to the carbon atom.


Combined 13C and 1H Spectra


Differences Between 1H and 13C Technique

• Resonance frequency is about one-fourth that of hydrogen, 15.1 MHz instead of 60 MHz.

• Peak areas are not proportional to number of carbons.

• Carbon atoms with more hydrogens absorb more strongly.


Spin-Spin Splitting

• It is unlikely that a 13C would be adjacent to another 13C, so splitting by carbon is negligible.

• 13C will magnetically couple with attached protons and adjacent protons.

• These complex splitting patterns are difficult to interpret.


Proton Spin Decoupling

• To simplify the spectrum, protons are continuously irradiated with broadband “noise,” so they are rapidly flipping.

• The carbon nuclei see an average of all the possible proton spin states.

• Thus, each different kind of carbon gives a single, unsplit peak because carbon-hydrogen splitting was eliminated.


1H and 13C-NMR of 1,2,2-Trichloropropane


Off-Resonance Decoupling 13C-NMR of 1,2,2-Trichloropropane

• 13C nuclei are split only by the protons attached directly to them.

• The N + 1 rule applies: a carbon with N number of protons gives a signal with N + 1 peaks.


Interpreting 13C NMR

• The number of different signals indicates the number of different kinds of carbon.

• The location (chemical shift) indicates the type of functional group.

• The splitting pattern of off-resonance decoupled spectrum indicates the number of protons attached to the carbon.


Two 13C NMR Spectra


DEPT 13C NMR

• DEPT (Distortionless Enhanced Polarization Transfer) provides the same information as off-resonance decoupling.

• Easier to run with Fourier-transform spectrometers.

• Each 13C nucleus is magnetically coupled to the protons bonded to it.


Information from 13C DEPT


Magnetic Resonance Imaging (MRI)

• Nuclear magnetic resonance imaging is a noninvasive diagnostic tool.

• “Nuclear” is omitted because of public’s fear that it would be radioactive.

• Computer puts together “slices” to get 3-D images.

• Tumors are readily detected.


MRI Scan of a Human Brain

• MRI scan of a human brain showing a metastatic tumor in one hemisphere.


MRI Image of the Pelvic Region

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